LONDON — A clinical trial of an experimental vaccine against the deadly Ebola virus is set to start shortly, according to British drugmaker GlaxoSmithKline, which is co-developing the product with U.S. scientists.

The world’s worst outbreak of Ebola has killed nearly 1,000 people in West Africa, and the disease could continue spreading for months, increasing pressure on researchers to accelerate their work on new medical interventions.

There is no proven cure or vaccine to prevent infection with Ebola, and the scale of the outbreak has prompted the World Health Organization to declare it an international health emergency.

GSK’s experimental vaccine has already produced promising results in animal studies involving primates, and it is due to enter initial Phase I testing in humans, pending approval from the U.S. Food and Drug Administration.

A company spokeswoman said Sunday that the trial should get underway “later this year,” while GSK’s partner the U.S. National Institute of Allergy and Infectious Diseases said in a statement on its website it would start “as early as fall 2014,” implying a potential September launch of testing.

Even if is fast-tracked, however, and emergency procedures are put in place, the new vaccine could not be ready for widespread deployment before 2015 — even assuming it works as well as hoped.

“It is right at the beginning of the development journey and still has a very long way to go,” the GSK official said, declining to be drawn on a possible timeline for launch.

The investigational vaccine is based on a chimpanzee adenovirus into which two Ebola genes have been inserted, which means it contains no infectious Ebola virus material. Adenoviruses are best known for causing the common cold.

Once the vaccine enters a cell and delivers its genetic payload, the two gene inserts produce a protein that generates an immune response in the body — but the adenovirus carrying the genes does not replicate further.

GSK acquired the vaccine after buying Swiss-based biotech company Okairos for $335 million last year.

The U.S. NIAID — part of the National Institutes of Health — also is supporting work on other early-stage Ebola vaccines, including one from Johnson & Johnson’s Crucell unit that should enter Phase I clinical testing in late 2015 or early 2016.

The Crucell vaccine is designed to give additional protection against Marburg, another severe and highly fatal disease caused by a virus from the same family as Ebola.